Gaseous discharge lamps generally comprise a transparent or translucent quartz envelope containing a gas or vapor in an otherwise evacuated enclosure which upon energization glows to provide an intense light. The vapor may be a metallic vapor such as that produced by a small amount of mercury in the evacuated enclosure. Thus, upon energization the mercury forms a vapor which is ionized to create an intense glow. One method of energization of the lamp is provided by using electrodes which extend into the enclosure. A potential connected across the ends of the electrodes external to the envelope effects the glow.
Lamps of the type described find particular use in mask projection systems of the type disclosed in U.S. Pat. No. 4,011,011, having the same Assignee as the present invention. This patent describes a mask projection system wherein photoresist coated wafers are exposed to the light projection of a mask containing intricate circuit details in the production of integrated circuits. A lamp used in such an environment must be precisely positioned in the optical projection system. Thus, assuming an accurately positioned mounting arrangement the lamp itself must be dimensionally accurate in its configuration.
A critical consideration in lamps of this nature is the security of the connection between base and envelope. Cost efficiency gained by simplification of fabrication procedures is also another important factor of the present invention. A lamp manufactured for use in the mask projection system described in the above mentioned patent is disclosed and claimed in U.S. Pat. No. Re. 30,315 having the same Assignee as the present invention. This patent describes a lamp having a glass envelope defining in a cavity of capillary cross section and arcuate configuration. One end of the envelope has an electrically conductive mounting base fixed thereto whose configuration and relationship to the rest of the envelope is such that upon installation of the base and a mounting fixture, the lamp is accurately positioned in its operating environment.
In the manufacture of the lamp, the metal base has an axial bore sufficiently large to receive an end stub of the envelope. The base and envelope are positioned in all 6 degrees of freedom axially, radially, and angularly relative to each other with the aid of a jig. An epoxy is then inserted in the annular space defined by the outside diameter of the lamp and the inside bore diameter of the base. The epoxy is allowed to harden and may be aided in the hardening process by heating. This type of mounting requires that the glass envelope be bent to its arcuate configuration prior to basing which significantly increases the steps to produce a completed lamp.
In addition, aside from the time consuming steps required to position and fix the metal base on the glass envelope, the use of epoxy results in a 2 to 3% rejection rate of the lamps due to loose bases.
Also, a defective batch of epoxy can result in a reworking of an entire day's production run.
The present invention is an improvement of the above described invention of U.S. Pat. No. Re. 30,315 by utilizing a compression fit of the base to the lamp which eliminates the use of the epoxy sealing technique with all its attendant disadvantages. In addition, the present invention provides an extremely secure fit between base and envelope which is obtainable in a highly cost efficient manner.